Nondestructive Inspection of Graphite-Epoxy Composites for Heat Damage Using Laser-Induced Fluorescence

Abstract

Laser-induced fluorescence has been applied to the nondestructive inspection of graphite-fiber-reinforced polymer-matrix composites. Upon curing, the epoxy resin in these materials forms a highly cross-linked network of aromatic sub-units. This result yields a material with significant absorptivity from the ultraviolet through the near-infrared and a fluorescence bandwidth covering 200 to 300 nm. When subjected to temperatures beyond the recommended upper service temperature, these materials experience thermally induced degradation and a reduction in mechanical strength. Furthermore, these heat-damaged materials exhibit significant changes in fluorescence intensity and in the wavelength of maximum fluorescence intensity. These spectroscopic changes can be correlated with the extent of heat damage. An imaging approach has been developed to monitor these fluorescence signatures, allowing rapid nondestructive inspection of large surface areas. This capability provides qualitative and quantitative information about the location and severity of damage.

Keywords

Fiber-reinforced composites PMCs Laser-induced fluorescence LIF Thermal damage NDE

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